Referring to FIG. 1, currently known and utilized image sensors use a pixel 10 which is a pinned-photodiode-pixel CMOS image sensor. It incorporates a photodiode 20 readout through a transfer gate 30 onto a sense node 40 which can be reset to a reference voltage and which is connected to a readout, for example, the gate of a source follower circuit 50. In addition, the photodiode 20 is connected to an overflow gate 60 to carry off photocurrent generated during the time outside of the shutter window.
The photodiode 20 is made as a pinned photodiode with a shallow 20 p-type pinning layer 70 and a deeper n-type diffusion 80 to create a buried channel with controlled potential, for example 1.5 V, for holding charge. This combination of dopings produces a very low dark current and density of dark current point defects. An overflow drain 90 and a reference bias connected to the reset gate are set to a higher voltage, for example 3.3 V.
Referring to FIGS. 1 and 2, operation involves using either the overflow drain 90 or the transfer gate 30 and the reset gate 100 combination to empty the photodiode 20. The reset gate I 00 also is used to reset the sense node 40 to a set bias greater than that of the photodiode 20. The overflow drain 90, the transfer gate 30, and the overflow gate 60 are then turned off on all pixels simultaneously to begin the shutter window and the signal from the incident light is integrated on the photodiode 20. To end the shutter window, the transfer gate 30 is turned on and the signal charge is completely transferred to the sense node 40. The overflow drain 90 is turned on so that any additional charge generated on the photodiode 20 will be drained away. The output is then readout; the reset gate 100 is used to set the sense node 40 back to the reference voltage, and the output is readout again. The signal is the difference between the two readouts.
The prior art has several limitations as follows: (1) the reference readout must be done after the signal readout so that they are from uncorrelated reset operations and temporal noise is added; (2) the signal charge must be held on the sense node from the end of the shutter window until its readout, allowing for the buildup of dark current as background and as bright points during this time.
Consequently, a need exists for overcoming the above-described drawbacks.